1. Field of the Invention
The present invention relates to an apparatus for controlling an air intake system of an internal combustion engine. More particularly, it relates to an apparatus for controlling an air intake system having a variable effective length.
2. Description of the Related Art
This invention is directed to an apparatus for controlling a variable-effective-length air intake system disclosed in a copending U.S. patent application Ser. No. 805,740 filed Dec. 6, 1985 now U.S. Pat. No. 4,738,229 and assigned to the assignee of the present invention. The air intake system disclosed therein comprises a surge tank, the inside of which is divided by a partition wall into two elongated parallel volumetric chambers. The surge tank has a portion extending beyond the branch tube located at a position most remote from the air inlet of the surge tank. The partition wall of the surge tank extension is provided with an opening communicating the volumetric chambers with each other. This opening is opened and closed by a control valve operated by an actuator in response to varying engine operating conditions. When the control valve is closed to isolate the two volumetric chambers of the surge tank from each other, the air intake system operates as if the effective length thereof is elongated to match a lower resonance frequency of the air column in the intake system, thereby increasing the flow rate of the intake air and generating a high output torque during a lower engine speed. Conversely, when the control valve is opened to communicate the volumetric chambers with each other, the effective length of the air intake system in reduced, thereby reducing the flow resistance through the system and improving the power at high speed operation of the engine.
The control valve is controlled, for example, by a vacuum actuator which, in turn, is controlled by an electronic control unit. The effective length of the air intake system is controlled, in the first place, in accordance with the rotational speed of the engine, in such a manner that, during high speed operation of the engine, the effective length is reduced and, during low speed operation, the effective length is extended as mentioned above. During the low speed condition, however, it is desirable to reduce the effective length of the air intake system when the engine is running under a light load condition, in order to reduce the flow rate of the intake air and, hence, the fuel comsumption. Accordingly, in the second place, the effective length of the air intake system is controlled depending on the engine load.
Detection of the engine load may be performed in various ways. For example, manifold vacuum may be used, or in another method, the engine load is decided on the basis of the amount of intake air drawn into the engine per one revolution of the engine. Alternatively, the throttle valve opening may be used as a parameter reflecting the engine load. The manifold vacuum and the amount of intake air per revolution are capable of adequately accurately reflecting the engine load during the steady load condition but have a disadvantage in that their responsiveness to the transitional condition of the engine load is so poor that an unacceptable delay is caused in the control of the effective length of the intake system. The throttle valve opening will quickly respond to a rapid change in the engine load but will not accurately refect the engine load during a steady load condition.